DESCRIPTION: The overall goal of the principal investigator's research program is to understand the role of protein phosphatases in signal transduction pathways. Protein phosphatase type 1 (PP1) constitutes a major proportion of serine/threonine phosphatase activity in the cell. Four PP1 holoenzymes have been identified and all contain a similar catalytic subunit (CS1) but differ in the associated regulatory subunits. The function of the latter components appear to be to target the enzyme to various cellular compartments, to confer substrate specificity and to control enzyme activity. Biochemical and genetic studies have provided evidence that PP1 is involved in several cell functions, including glycogen metabolism, calcium transport, muscle contraction, gene expression, cell cycle and cell growth. Although much has been learned about the participation of these enzymes in a variety of cellular processes, knowledge of their intracellular targets and regulation of their activity is quite limited. The objectives of this project are to understand the molecular basis of the interaction between catalytic and regulatory components, to elucidate the regulatory mechanisms and to define the role in specific cell functions of two forms of type 1 holoenzyme: the ATP-Mg-dependent and the glycogen/sarcoplasmic reticulum-associated protein phosphatases. The ATP-dependent phosphatase is a cytosolic enzyme and contains inhibitor-2 as the regulatory subunit, whereas the glycogen/SR- associated form interacts with both membranes and glycogen via its regulatory component RGL. Structure/function studies will be directed at characterizing the functional domains of CS1 and the regions of interaction between CS1 and the regulatory subunits, I-2 and RGL, by utilizing the yeast two-hybrid system and in vitro reconstitution assays of bacterially expressed mutant proteins. Studies of the roles of the two phosphatases in cells will involve overexpression of CS1 together with wild type or mutated regulatory subunits (I-2 or RGL) in mammalian cells to; analyze any modulation of cellular responses to extracellular signals. In addition, the control of the phosphatases will be probed by overexpressing active or dominant negative versions of putative upstream regulatory elements such as those in the MAP kinase/ERK cascades. Success in this investigation should lead to improved understanding of the physiological role of some of the major type 1 phosphatases.